Okay, this is definitely a behavioral issue. What kind of training have you done with him? And have you cleaned everything with a good enzymatic cleaner that will eliminate both any scent left by the chihuahua and and scent of his own? What is your daily routine with him? You need to become the one in charge, and take him outside and keep him outside, for a long walk if necessary, until he "empties the tank," and work on some obedience training. It need not be expensive, you can do it yourself, get a library book if necessary.